Machinery parts and mechanical component parts such as seal and piston rings can be made using powder molding techniques such as compression molding. Moldable powders can be prepared from materials such as metals, ceramics, or polymers.
Polymers with very high melting or softening points do not flow readily, and therefore are not generally considered to be injection moldable. Often, powder molding techniques are the preferable method of manufacturing shaped parts from such polymers. However, because a non-melting powder typically does not flow and distribute itself in the mold as readily as a meltable, or thermoplastic polymer, features such as fine separations or wider gaps can be difficult to mold into the part. An example of a part requiring a separation would be a seal ring, where the separation allows the ring to be opened and placed on a shaft, or to allow for thermal expansion. Hereinafter, the terms “separation”, “gap”, and “joint” may be used interchangeably.
High temperature resins are increasingly replacing metals in the fabrication of machinery parts and mechanical components. As a result, significant reductions in production and replacement costs for the machinery parts and mechanical components have been realized. To replace metals in machinery parts and mechanical components, the high temperature resins should have high resistance to mechanical wear, surface stress, and extreme temperature conditions. Additionally, the performance characteristics of the high temperature resins should equal or exceed that of the metals being replaced.
Polyimides are particularly preferable high temperature resins because of their mechanical strength, dimensional stability, thermal stability, chemical stability, flame retardance, and dielectric properties. Polyimides, such as those described in U.S. Pat. No. 3,179,614 issued to Edwards on Apr. 20, 1965, can be used in a wide variety of commercial applications. The performance characteristics of these polymers under stress and at high temperatures have made them useful as bushings, seals, electrical insulators, thermal insulators, compressor vanes and impellers, pistons and piston rings, gears, thread guides, cams, brake linings, and clutch faces.
A desirable group of polymers suitable for use are those that retain desirable mechanical properties at high temperatures. Polymers in this group, however, often melt at very high temperatures or decompose without melting. In addition, their viscosities in the melt phase are extremely high. Therefore, these polymers are considered to be intractable, that is, non-melt processible. Thus, forming these polymers into shaped articles is expensive at best and difficult in many cases. For example, nylons of hexamethylene diamine and terephthalic acid exhibit excellent temperature resistance but cannot be melt-spun or molded because they decompose before their crystalline melting temperatures are reached. Similarly, many other substantially aromatic polymers such as polyimides of pyromellitic anhydride and aromatic diamines cannot be melt processed. Powder processing and sintering techniques have been used to process such intractable polymers into useable articles. Thus, in the context of the present application, “non-melt processible” refers to resin particulates that either have a melting transition temperature (“Tm”) of at least 260° C., in the case of resin particulates that have a discernable melting point, or have no discernable melting point but are stable in temperatures up to at least 260° C.
Sealing rings have been made from a variety of materials, most commonly from metals such as cast iron, and various polymers. Polymers with good high temperature properties, low frictional coefficients, and reduced wear resistance have been found to be particularly useful in sealing ring applications. Since the ring is placed on a piston or shaft, and the ring material is often inelastic, a separation must be placed in the ring to facilitate application of and removal from the piston or shaft. The separation also allows thermal expansion and contraction of the seal ring. Sealing rings are used in mechanical devices for creating a seal between a shaft or rod and a bore, as for example in compressors, automatic transmissions, and power steering devices. A seal ring is generally of an open annular shape and can be mounted on a circumferential groove of a shaft or rod that is situated within a cylindrical housing. The function of the seal ring is normally to control the leakage of fluid from one side of the ring to the other side while allowing the shaft or rod to turn or pulsate within the cylindrical housing. Seal rings have been made with joints that allow the rings to expand or contract with expansion or contraction of the shaft or rod on which the seal ring is mounted, as occurs for example during thermal expansion or contraction of the shaft. The joints of such expandable seal rings have been made with various geometrical configurations and are generally a compromise between the functional requirements and the affordability of the ring. In most all designs it is preferable for the open separation between ends of the ring to be minimized when operating in the housing. Commonly known joint arrangements for seal rings include butt joints, scarf joints, and step joints. Some applications for these sealing rings are compressors, pumps, automatic transmissions, and power steering devices. The known methods for preparing separations in these rings are direct forming, machining or fracturing. Machining of such rings has been both tedious and labor intensive, resulting in higher part manufacturing costs. Furthermore, when a ring has been machined, material is actually removed from the ring such that when the machined edges are brought back into contact with each other, the ring is then “out of round” that is, no longer circular.
A semi-rigid ring, such as a Vespel® ring cannot be stretched over a shaft so as to be placed in a ring groove on said shaft. The ring, having the purpose of sealing a fluid, such as transmission fluid, has a high pressure side and a low pressure side. The ring, properly seated in the groove, should provide a seal, thus restraining the high pressure fluid from freely passing through the ring to the low pressure side. But in order to install the semi-rigid ring, the ring must have a separation to allow the ring to be spread (an increase in the effective diameter of the ring) so as to pass over the large shaft diameter and be placed in the smaller shaft diameter of the ring groove. A preferred ring separation is one that allows for ring diameter expansion/contraction in response to changing environmental conditions. A preferred ring separation would also maintain a constant sealing capability over the entire range of conditions it operates under. A preferred separation is one that has overlapping segments, such as in a step gap joint or scarf joint ring.
In U.S. Pat. No. 3,720,418 to Berg, a method for fracturing a ring is described wherein a “notch” is first etched or scored into the outer surface of a ring, and then the notched area is struck with a heavy object to create the fracture. U.S. Pat. No. 5,988,649 to Van Ryper et al. discloses a seal ring having a fracture line through its thickness to form opposing faces. U.S. 2005/0156004 to Edwards, discloses an improved fracturing device using a recessed pocket instead of support pins.
Parts with voids such as cavities, channels, chambers, etc. are typically produced by machining which again is tedious and labor intensive and results in higher part manufacturing costs.
It can be desirable to have a method for forming separations and voids in powder molded parts during a molding process. It can further be desirable to have an efficient method for forming separations in powder molded seal rings.